Removing carbon bisulfide from viscose fiber with a liquid seal vacuum pump



March 30, 1954 c HALL 2,673,785

REMOVING CARBON BISULFIDE FROM VISCOSE FIBER WITH A LIQUID SEAL VACUUM PUMP Filed April 23, 1951 RECOVERY PRESSURE SWITCH RECOVERED JSEPARATOR SOLENOID1 I I VALVE VENT MAKE UP WA TER SEAIE WATER ST 0 R A gs E 1/20 TAN K j 22 H E INVENTOR.

EXCHANGER Bfllg yrorz Chariesflall WMZM CONTROL WA TE R A TTORNE Y.

Patented Mar. 30, 1954 REMOVING CARBON BISULFIDE FROM VIS- COSE FIBER WITH A LIQUID SEAL VACUUM PUMP Myron Charles Hall, Snyder, N. Y., assignor to E. I. du Pont de Nemours& Company, Wil- .mington, Del., a corporation of Delaware Application April 23, 1951, Serial No. 222,397

Claims.

This invention relates to aprocess for recovering carbon bisulfide from wet acid, gel yarn, filaments, tow or the like produced from viscose by the usual acid-salt bath spinning process.

Numerous methods had been proposed and used for the recovery of carbon bisulfide from wet acid regenerated cellulose yarn and the like made by the viscose process. Many of the prior proposals involve hot-water washing, which is effective but costly in view of the large volume of hot water needed. While it is true that regenerated cellulose yarns can be washed free of not only carbon bisulfide but acid and salt impurities more rapidly with hot water, the time saved is not justified by the additional cost of heating this water. On the other hand, when relatively cool water is used to wash freshly formed regenerated cellulose products, uniform removal of acid and salts is frequently unobtainable in a reasonable time due to the presence of carbon bisulfide which, being insoluble in water andbeing below its boiling point, is only very slowly dissipated or overcome as a barrier. Furthermore when by using long cycles the removal of acid and salt is completed, the amount of CS2 remaining is commonly too high to permit processing without elaborate ventilation. could be economically removed, there would be no serious obstacle to successfully washing the regenerated cellulose structure with cool water to free it from mineral acid, salts, and otherbyproduct impurities.

If the carbon bisulfide Generally about 16 to 18 per cent carbon bisulflde remains in the gel yarn or tow as it is collected from theacid spinning bath, based on the dry weight of the regenerated cellulose. Unless this carbon bisulfide is removed down to a value of about four percent or less'based on the dry weight of the cellulose, --difficulty will be expe'i rienced in uniformly washing the regeneratedcellulose product unless the wash water is heated to above the boiling point of the carbon'bisul- J fide.

It has been found that a practical solu- I tion of this problem is to remove the c'arbonbibon bisulfide, such as from 30 to C.,. can be readily and efiectively carried out. a It is, therefore, an object of this invention to provide an improved process for purifying and washing regenerated cellulose products produced by the viscose process. Another object of this invention is to provide an improved method for recovering carbon bisulfide from wet acid, gel, regenerated cellulose filaments, yarns, or tows spun from viscose. Still another object of this invention is to provide in combination a suitable carbon bisulfide recovery step followed by an effective washing step for freeing regenerated cellulose filament structures of by-product impurities. These and other objects will be more fully explained in the discussion which follows:

In general the objects of this invention are accomplished by placing the freshly-spun viscose filament product in a sealed chamber and subjecting the same to evacuation by means of a liquid seal pump. The sealing liquid for this pump is maintained at a temperature above the boiling point of the carbon bisulfide to prevent any substantial condensation of the carbon bisulfide thereby.

The liquid seal pump mentioned which is sometimes referred to as a liquid-ring pump, is a ring pump containing an annular body of liquid, formed under the action of centrifugal force, and alternately entering channels formed in the pump casing and in the wings. This type of pump has been used heretofore in connection with carbon bisulfide recovery from viscose rayon products, which process is described in U. S. Patent 2,196,843. In this patent the liquid-seal pump is used in the usual manner, the sealing water being at a relatively low temperature and the carbon bisulfide being condensed thereby. Condensation-of the carbon bisulfide in the seal water poses a number of difiicult problems: Emulsification of the carbon bisulfide in the water requires the addition of chemical agents to break the emulsion; contact of the condensed carbon bisulfide with metal'parts of the pump causes corrosion difficulties; and, probably most important of all, an excessive amount of refrigeration is neededtto maintain the seal water at the necessary low temperature to efiiectively carry out this liquid phase method.

It is generally known that about of the power input to a liquid seal vacuum pump goes into heating the seal water. It would, therefore, seem desirable to operate the pump with seal water at some elevated temperature, which could easily be maintained without excessive cooling of the recirculated seal water. Nevertheless, this idea is directly opposed to the generally accepted and well established practice-of using these liquid seal vacuum pumps in thatt he maximum pumping eificiencycan only be realized when relatively cold seal liquor is'used. Normally, operating the pump at relatively low pumping efficiency to produce a minimum absolute pressure no less than about 7 or 6 inches of mercury would indeed seem foolish. In fact, even trying out this system was frowned upon by a number of technical associates of the inventor as being wasteful in time and money; and it might have proved so if some unusual results had not been observed and taken into account in arriving at preferred conditions of operation. For instance, it was found that a relatively poor vacuum was desirable under the conditions obtained, in which the acid yarn or tow was collected as a 33 inch layer in a plurality of compartments in a large rectangular can equipped with a false bottom and was charged into the evacuation chamber at about 38 C. In order to evaporate the carbon bisulfide readily, a relatively high temperature is desirable. It was found that a strong vacuum caused excessive evaporation of water from the rayon tow, with substantial cooling of the yarn mass, and consumed much of the pump capacity with water vapor rather than carbon bisulfide vapor.

Surprising as it may seem, more carbon bisulfide could be evaporated per hour at an absolute pressure of -10 inches of mercury, and preferably at a pressure between 5 /2 and 7 inches of mercury, than could be obtained with the customary high vacuum of about 2 inches of mercury absolute pressure or lower. This was possible not only because the reduced vacuum removed less water Vapor but, in view of less water being evaporated, the temperature of the acid regenerated cellulose mass was not appreciably lowered and carbon bisulfide removal was more rapid due to the temperature advantage. At' the same time the relatively high temperature of the seal water, which automatically controls the vacuum to cause a high ratio of carbon bisulfide vapor to pass into the pump, caused the carbon bisulfide to stay in the vapor state. This vapor was passed on and separated as an air-carbon bisulfide vapor mixture very readily, permitting easy recovery as will be explained hereinafter. Maintaining the seal water temperature at or around 50 C. may in some cases require only a simple circulating system to utilize heat generated by the pump, excess heat being lost to the atmosphere. However, if use can be made of this low temperature heat, a suitable heat exchanger may be set up and the seal water passed through it.

The single figure of the drawing illustrates by a flow sheet a suitable system for utilizing the present invention.

The wet acid regenerated cellulose product-to be freed of carbon bisulfide is placed in vacuum chamber In. The chamber is connected to liquid seal vacuum pump ll. Water is'supplied to the pump from heat exchanger l2at about 50 C., the flow being adjusted by a solenoid-operated valve I3, to maintain a pressure of 5 to 7 inches of mercury absolute in the cell. Water and carbon bisulfide vapor pass to a separator I 4, and separated vapor is passed to a carbon bisulfide recovery system, indicated at l5. Flow of vapor is controlled by a pressure switch I 6 and an automatic valve l7.

Water from the separator flows to storage tank 20. The water has been heated by passage through the pump and some cooling occurs during the time it remains in the storage tank, Water is added to the tank at 2| to make up for evaporation occurring in the pump. From the tank the water passes to a centrifugal pump 22 4 and is returned to vacuum pump I I through heat exchanger I 2. The temperature is adjusted to about 50 C. during passage through the heat exchanger by means of control water. The flow of control water is regulated by valve 23. A valve 25 is provided in the linefrom the vacuum chamber to the vacuum pump for use when the chamber is opened to remove a charge of regenerated cellulose and add a new charge.

To further illustrate the invention, the following specific example is given: Rayon filament tow is collected at the spinning machine in a large rectangular can designed to hold about 600 lbs. of production, figured on an air dry basis. This amounts to about 3,000 lbs. of wet acid tow as collected. The can is divided into three equal compartments, and a false bottom of screen material is located about 2" above the true bottom to allow the escape of CS2 vapor. The tow is allowed to drop into this can to a total depth of about 33", and evaporation of CS2, air and water takes place from only the top and bottom surfaces. The tow, as collected, will contain about 16 to 18% carbon bisulfide, based on the dry weight of the cellulose, or in one can of production there will be about 100 pounds of carbon bisulfide. The can and its contents are charged into vacuum chamber I0, which is then sealed and the liquid seal pump ll, connected therewith is started. Varying the size of charge and varying the vacuum within the chamber by changing the temperature of the seal water gave the following results:

A B C D Staple Tow Temp. O.:

Start 37 38 38 37. 5 End 30 31 30 32. 5 Evacuation Time, minutes 9O 90 90 Minimum absolute Pressure, Inches Hg 2.0 5. 7 2. 4 5. 5 Weight sample (in pounds-dry basis) 600 600 300 277 Percent CS1 in sample after evacuation. 4. 4 2. 9 2. 4 l. 4 Treatment (based on dry sample) Seal Water, Temp. G 5 50 Percent CS1 in Wash Efilucnt end 1 hr. wash .054 .037 end 2 hr. wash 037 022 end 2% hr. wash 022 .015

1 Recirculated water.

As will be noted from the data above using the preferred conditions of this invention, i. e., seal water at 50 C., over 90% of the carbon bisulfide contained in the acid gel tow charged into the vacuum chamber wa removed by the end of oneand-one-half hours. At the end of this time the absolute pressure in the chamber was not below 5 inches of mercury, which was the minimum pressure obtained.

Following this evacuation treatment the can 01 rayon towwas removed from the chamber and located in a wash area under suitable sprays, and water at 40 C. and at a pH of 6.75 was showered onto the top of the tow at the rate of about 15 gallons per minute. If free drainage were permitted and the water allowed to run off from underneath the screen as fast as it could. no great depth of water could be maintained in the can, and poor washing efliciency would result. It was therefore found desirable as a result of numerous tests to install a standpipe opening into the bottom chamber of the can and extending above the bottom of the can about 15 inches, or substantially half the depth of the tow contained in the can. This being the only run-off for the water, the discharge rate did not reach the rate of water application to the can until the can was substantially filled and the surface of the water was above the top layer of tow. At about position equilibrium was established between the and so that this level of wash water maintained throughout most of of the cycle. if the porosity of the charge too little to maintain the wate level at the desired point, a suitable control can be installed to .iaintain the liquid level within the desired ts and wi'hcut overfiowing the can. The w cycle i continued for five hours at which the spray was shut off and the can tow removed for further processing. While the carbon bisulfide remaining in the tow evacuation was the range of from about 1.5

to 4%, ba ed on the air-dry weight of the tow, at the ex five hour washing treatment the carbon contained in the efzluent water was ab 0.081%, the carbon bisulfide conoentrat on in the air above the tank after this pu 'ification was of the order of about three parts per million, which is considered very satisfactory.

By this invention it is possible to economically remove carbon bisulfide from wet acie masses of viscose process filaments to the extent of at least 75 or 89% of that originally contained in the acid yarns, and to a sufficiently low value that washing can be effectively carried out with wash water at a temperature substantially below the boiling point of carbon bisulfide. Condensation and final recovery of the carbon bisulfide vapors from the discharge of the Water-seal pump is carried out by simple, known methods without need of excessive refrigeration, and because the carbon bisulfide passes through the liquid seal pump in the vapor state, it has a surprisingly low corrosive effect on the pump parts.

Since many dififerent embodiments of the invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited by the specific illustrations except to the extent defined in the following claims.

What is claimed is:

1. In a process for purifying freshly spun regenerated cellulose fibers which includes the steps of placing the regenerated cellulose fibers a closed chamber, then reducing the pressure in the chamber with a liquid seal pump to evaporate carbon bisulfide from the fibers, then removing the regenerated cellulose fibers from the chamber and washing to remove acid and salt, the improvement which comprises maintaining the pump seal water at a temperature around 50 C. to keep the carbon bisulfide in vapor form in the pump discharge and evaporating the carbon bisulfide from the regenerated cellulose fibers an absolute pressure of 5 to 10 inches of mer cury to reduce the carbon bisulfide content of the fibers to less than 4% based on the dry weight of the cellulose, and then washing the regenerated cellulose fibers with wash water at a temperature substantially below the boiling point of carbon bisulfide.

discharged from pump, and recirculatpainted water to the pump for reuse as seal water.

vapcr and recoverin the carbon bisulfide, then regulating the separated water to a temperature of about 6., and recirculating the water to the pump for use as seal water.

i. In a process for purifying regenerated cellulose fibers produced from viscose by the conventional acid salt bath spinning process using a liquid seal pump to provide reduced pressure for evaporating carbon loisulfide from the freshly spun fibers, the improvement which comprises evaporating carbon bisulfide from the freshly spun regenerated cellulose fibers at an absolute pressure of 5 to 19 inches of mercu y while maintaining the temperature of the p np seal water above the boiling point of the carbon bisulfide until the amount of carbon bisulfide remaining in the fibers is less than a percent based on the dry weight of the cellulose, and then washing the fibers with wash water at a temperature substantially below the boiling point of carbon bisulfide until substantially all of the acid, salt and remaining carbon bisulfide are removed.

5. In a process for purifying freshly spun regenerated cellulose fibers which includes the steps of placing regenerated cellulose fibers in a closed chamber, then reducing the pressure in the chamber with a liquid seal pump to evaporate carbon bisulfide from the fibers, then removing the regenerated cellulose fibers from the chamber and washing to remove acid and salt, the improvement which comprises maintaining the pump seal water at a temperature around 50 C. to keep the carbon bisulfiole in vapor form in the pump discharge and evaporating the carbon bisulfide from the regenerated cellulose fibers at an absolute pressure of between 5 /2 and 7 inches of mercury until the carbon bisulfide remaining amounts to 1.5% to 4% based on the dry weight of the cellulose, and then washing the regenerated cellulose fibers with wash water at 30 C. to 45 C. until substantially all of the acid, salt and remaining carbon bisulfide are removed.

lVIYRON CHARLES HALL.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date Re. 20,079 Moritz et al. Aug. 25, 1936 1,579,711 Kampf Apr. 6, 1926 1,907,101 Funcke et al. May 2, 1933 2,035,981 Richter Mar. 11, 1936 2,1332% Baumert et al. Oct. 11, 1938 2,196,843 Van Deinse Apr. 9, 1940 

1. IN A PROCESS FOR PURIFYING FRESHLY SPUN REGENERATED CELLULOSE FIBERS WHICH INCLUDES THE STEPS OF PLACING THE REGENERATED CALLULOSE FIBERS IN A CLOSED CHAMBER, THEN REDUCING THE PRESSURE IN THE CHAMBER WITH A LIQUID SEAL PUMP TO EVAPORATE CARBON BISULFIDE FROM THE FIBERS, THEN REMOVING THE REGENERATED CELLULOSE FIBERS FROM THE CHAMBER AND WASHING TO REMOVE ACID AND SALT, THE IMPROVEMENT WHICH COMPRISES MAINTAINING THE PUMP SEAL WATER AT A TEMPERATURE AROUND 50* C. TO KEEP THE CARBON BISULFIDE IN VAPOR FORM IN THE PUMP DISCHARGE AND EVAPORATING THE CARBON BISULFIDE FROM THE REGENERATED CELLULOSE FIBERS AT AN ABSOLUTE PRESSURE OF 5 TO 10 INCHES OF MERCURY TO REDUCE THE CARBON BISULFIDE CONTENT OF THE FIBERS TO LESS THAN 4% BASED ON THE DRY WEIGHT OF THE CELLULOSE, AND THEN WASHING THE REGENERATED CELLULOSE FIBERS WITH WASH WATER AT A TEMPERATURE SUBSTANTIALLY BELOW THE BOILING POINT OF CARBON BISULFIDE. 